This invention relates to the separation of the components of mixtures of nitrogen and hydrocarbons, particularly mixtures of nitrogen and methane, such as natural gas, by low temperature rectification, and is particularly concerned with procedure for separation of methane containing only a small amount of nitrogen, from mixtures thereof with nitrogen, and wherein the nitrogen concentration can vary over a wide range, e.g. from 15% to more than 80%, employing non-adiabatic or differential distillation within the fractionating zone for maximum thermodynamic efficiency, and utilizing gas feed pressures significantly below the critical pressure of the mixture, e.g. below about 700 psia, thus eliminating the need for compression above natural gas reservoir pressures, and only requiring recompression of the methane or hydrocarbon gas product to the desired delivery pressure, thereby eliminating the need inieially to compress the waste nitrogen in the raw feed stock.
The separation of the components of natural gas containing low-boiling hydrocarbons and nitrogen, such as for example mixtures of methane and nitrogen as in natural gas to remove nitrogen and to recover hydrocarbon, e.g. methane, of substantially reduced nitrogen content, for use as a quality pipeline gas has become of considerable importance. It is thus desirable to provide a pipeline gas containing not more than 7% nitrogen from natural gas mixtures which can contain from 15% to more than 80% nitrogen, as efficiently as possible. It will be understood that all percentages of components of the gas compositions set forth herein are on a mol basis unless otherwise indicated.
U.S. Pat. Nos. 3,568,459 and 3,589,137 disclose processes and systems for the separation of nitrogen from mixtures of nitrogen and hydrocarbons such as methane, by low temperature rectification. However, the processes of these patents require initial compression of the feed gas mixture to high pressures of the order of about 1000 psi, and the feed gas is throttled prior to introduction into the fractionating column, which is operated at a pressure, e.g. ranging from about 100 to about 300 psi. These processes utilize pumped liquid cycles in that the product hydrocarbon such as methane is withdrawn from the fractionating column as a liquid, which is then elevated in pressure by means of a mechanical pump, e.g. at a pressure up to about 75% or more of the pressure of the feed mixture, e.g. up to 800 psi, as noted in the 3,568,459 patent, and the resulting liquid product is then evaporated in countercurrent heat exchange with the raw feed mixture. This mode of procedure requiring high compression of feed gas and high compression of hydrocarbon product, imposes operational limitations upon these systems, which reduce their efficiency and render them economically disadvantageous for use in many low BTU gas reservoirs.
Thus, in order for the process of the above noted patents to function effectively, the raw feed gas stream must be at a pressure above the critical pressure of the mixture, e.g. in excess of 705 psi. In practice, it is desirable in such systems to maintain a feed gas pressure in excess of 800 psi and particularly above 1000 psi, since minor amounts of ethane, propane and heavier hydrocarbons normally found in natural gases, sufficiently alter the thermal properties of the feed mixture as to require higher feed pressures in order for heat exchange to occur.
Normal natural gas reservoirs are initially at high pressures but the pressure gradually declines over a period of time. When employing the processes of the above patents, compression of the entire feed stream from such gas reservoir would be required, including the waste nitrogen, to the above noted relatively high pressure. This limitation is expensive both in terms of capital expenditure and in operating energy. The process and system of the present invention are designed to operate efficiently with any feed pressure and with gas mixtures having a wide range of nitrogen concentration, requiring only that the feed pressure be say five to six times greater than the product evaporation pressure. This then requires only the recompression of the hydrocarbon or methane gas to the desired delivery pressure, e.g. about 300 to about 600 psia, and eliminating the need to compress the waste nitrogen.
Another limitation in the processes and systems of the above patents results from the above noted pumped liquid cycle employed by these patents. The critical pressure of a mixture is defined as the pressure above which only a single phase exists. Separation by fractionation and partial condensation can occur only in the presence of two phases, e.g. liquid and vapor. As a result, the systems of the above noted patents render it difficult economically to separate and recover heavier hydrocarbons such as propane, butane, and the like, which are frequently more valuable than the methane constituent.
U.S. Pat. No. 3,559,418 discloses liquefaction of natural gas containing nitrogen by low temperature rectification employing a natural gas feed at relatively low pressure, e.g. up to about 400 psia. However, the patent process is designed to liquefy the feed gas employing a reduced power requirement, regardless as to the nitrogen content of the final product, which can contain up to 15% nitrogen.